Infrared detector

ABSTRACT

An infrared photo-detector with multiple discrete regions of a first absorber material. These regions may have geometric shapes with sloped sidewalls. The detector also may include a second absorber region comprising a second absorber material that absorbs light of a shorter wavelength than the light absorbed by the multiple discrete absorber regions of the first absorber material. The geometric shapes may extend only through the first absorber material. Alternatively, the geometric shapes may extend partially into the second absorber region. The detector has a metal reflector coupled to the multiple discrete absorber regions. The detector also has a substrate containing the discrete absorber regions and the second absorber region. The substrate can further include geometric shaped features etched into the substrate, with those features formed on the side of the substrate opposite the side containing the discrete absorber regions and the second absorber region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.12/544,788 filed on Aug. 20, 2009.

TECHNICAL FIELD

An infrared detector which may be disposed in arrays of detectors for aninfrared focal-plane array imager.

BACKGROUND

Infrared imagers have many possible applications and can be used forvehicle collision avoidance, perimeter surveillance, engine diagnosisand monitoring, missile seeking, and large-area imagers. The discloseddual-band infrared photo-detector array and imager can operate at muchhigher temperature while still achieving high sensitivity. The higheroperating temperature (>150-200K) made possible by the disclosedinvention could greatly reduce the size and cost of the infrared imagerssince cryogenic cooling would not be needed. The higher temperatureoperation could reduce the cost of platforms containing such imagers andcould improve the reliability of their infrared sensors. The possibilityof operating at temperatures achievable with thermoelectric-coolersincreases the breadth of commercial applications since the imagers anddetector arrays would be more affordable.

Most prior detectors of infrared imagers have pixels that are defined byetching mesa structures that contain the light absorbing layer. FIG. 1 ashows the cross-sectional structure and FIG. 1 b shows a perspectiveview of a prior dual-band detector in which the mesa structure is formedby etching through the absorbing layer for one wavelength band but notetching the mesa through the absorbing structure for a second wavelengthband. See M. Vuillermet, F. Pistone and Y. Reibel, “Latest developmentsin MCT for next generation of infrared staring arrays,” ProceedingsSPIE, Vol. 7481, 2009, p. 74810F. In each pixel of this detector, thedetecting regions for the two bands are slightly offset from each other.Also, each diode detector of a given pixel has a separate electricalcontact, thereby allowing true simultaneous detection of light in thetwo wavelength bands. There is a layer comprising wider bandgap material(the NIR barrier) located between the two light-absorbing layers thatprevents electrical short-circuiting between the two diodes. Thisbarrier blocks both the electrons and the holes. In general, thelight-absorbing layer that absorbs the shorter-wavelength light islocated closer to the incident light, which is the layer located closerto the substrate in the case shown in the figure.

In the pixels of other prior detectors, the light absorbing regions forthe two wavelength bands are stacked directly above each other and bothof these light-absorbing layers are located within the same mesa. See G.Destefanis, et al., “Bi-color and dual-band HgCdTe infrared focal planearrays at DEFIR,” Proceedings SPIE Vol. 6206, 2006, p. 62060R. Thisstructure is illustrated in FIGS. 2 a and 2 b. Each pixel consists oftwo back-to-back N-on-P photodiodes. Each pixel has one independentcontact and also has a contact that is common to the other pixels of thearray. The two photodiodes of a pixel are electrically contacted andbiased through the single contact located on the top of the mesa andthrough the common contact located on the substrate side of thestructure. By switching the bias voltage for a pixel from a positivevalue to a negative value, it is possible to extract the photodetectedsignal from one or the other P/N junction photodiode.

We recently invented a single band detector (albeit having very widebandwidth) that has a reduce volume of its light-absorbing material.This detector contains multiple pyramid shaped features formed in eachpixel, with those pyramids located on the side of the detector facingthe incident light. See U.S. patent application Ser. No. 12/544,788filed on Aug. 20, 2009 and FIG. 6 thereof. The pyramids are etched intothe light absorbing layer. The pyramids are etched only partly throughthe light-absorbing layer so that there remains a physically continuousbase region of the light-absorbing layer to permit the majority carriersto be conducted to electrical contacts formed at the edges of thedetector array. This detector also contains mesas etched through theheavily doped collector or extractor layer of the P/N diode. The mesasare not formed in the main light-absorbing layer and the mesas face awayfrom the incident light. The pixel-specific electrical contact for agiven pixel is formed onto these collector mesas. The spatial extent ofa given pixel is defined by the electrical contact made to these mesas.There can be more than one mesa for each pixel and there are multiplepyramids in each pixel. As a variation of this detector structure, themesas can be made to have the shape of shallow pyramids. Those mesasalso can be coated with a metal reflector that conforms to the shape ofthe mesas.

In our prior detector of FIG. 3 herein, the light is incident from theside containing the pyramids rather than from the substrate side of thedetector. Thus, this detector is unlike the prior detectors illustratedin FIGS. 1 and 2. But a potential weakness of our prior detector anddetector array is that its substrate must be removed in order tofabricate its mesas and electrical contacts and to form the electricalconnections between that detector array and the electronic readoutcircuit. Thus the resultant thin detector-array structure of FIG. 3 ismore difficult to fabricate and to integrate with the readout circuit.

For the detectors of FIGS. 1 a, 1 b and 2, the pixel for thelonger-wavelength band is defined by the single mesa of that pixel. Forthe detector of FIG. 3, the pixel for the single band is defined by theextent of the electrical contact made to the one or more collector mesascomprising a pixel. The electrical contact made to the light-absorbinglayer is the common contact of the detector array. The pyramid-shapedregions of the light-absorbing layer do not specifically define theextent of a pixel but rather they extend throughout the light-facingsurface of the array.

Besides our prior detector of FIG. 3, there are no other infrareddetector structures to our knowledge that have pyramids or other suchgeometric shapes formed on both opposing sides of the detector. Thereare, however, prior solar cells that have pyramids or other surfacetexture etched into the substrate, with that textured surface facing theincident light (see FIG. 4). See A. Sinton, et al., “27.5 percentsilicon concentrator solar cells,” IEEE Electron Device Letters, Vol.EDL-7, no. 10, October 1986, p. 567. The textured surface reduces thefront-surface reflection of the incident light and improves the couplingof that incident light into the solar cell. Since the substrate materialof the solar cell absorbs the incident light, their pyramids forreducing the front-surface reflection of incident light are etchedpartially through the light-absorbing material of the solar cell.

There also are prior solar cells that have geometrically shaped featuresformed both on the side facing the incident light as well as on the sidefacing away from the incident light. See R. Brendel, et al., “Ultrathincrystalline silicon solar cells on glass substrates,” Applied PhysicsLetters, Vol. 70, no. 3, 20 Jan. 1997, p. 390. These geometric featuresare formed into both sides of a substrate such as glass, which istransparent to the light to be absorbed by the solar cell. Then alaterally continuous, or contiguous, film of light-absorbing material isformed onto one shaped side of the substrate (see FIG. 5). Solar cellsthat have pyramids or other geometrical shapes formed on both sides ofthe substrate were analyzed and found to have improved trapping andabsorption of the incident light. See Campbell and M. A. Green, “Lighttrapping properties of pyramidically textured surfaces,” Journal ofApplied Physics, Vol. 62, no. 1, July 1987, p. 243. For the solar cellsdescribed by Brendel et al., one set or both sets of electrical contactscan be formed on the side of the solar cell that faces away from theincident light. That side of the solar cell is covered with flat metalreflectors that also serve as electrical conductors to those electricalcontacts.

In other prior solar cells, geometric shapes such as pyramids are formedinto one side of a first slab of transparent conducting oxide material.One or two layers of thin, but laterally continuous, light-absorbingmaterials are formed above the shaped surface of the transparentconducting oxide. See P. Obermeyer, et al., “Advanced light trappingmanagement by diffractive interlayer for thin-film silicon solar cells,”Applied Physics Letters, Vol. 92, p. 181102 (2008) and C. Haase and H.Stiebig, “Thin-film silicon solar cells with efficient periodic lighttrapping texture,” Applied Physics Letters, Vol. 91, p. 061116 (2007).The side of the contiguous light-absorbing layers opposite that firsttransparent conducting oxide slab also has a pyramidal shape. Thatopposite side of the top-most absorbing layer is covered with a secondslab of transparent conducting oxide. This second slab of transparentconducting oxide has a shaped surface facing the light-absorbing layersand a flat surface facing away from the light-absorbing layers. The flatsurface of the second slab of transparent conducting oxide then can becovered with a metal reflector. For these solar cells, the electricalcontacts are formed on both opposing sides of the light-absorbing layerswith electrical connections provided by means of the transparentconducting oxide material.

Many high-sensitivity focal-plane photo-detector arrays for detectinglight at mid-wave infrared (MWIR) wavelengths or longer need to becooled to cryogenic temperatures (e.g., 77K and lower) in order tosufficiently reduce their internally produced noise current to levelsthat are below the background noise of the scene. However, cryogeniccoolers, such a Stirling coolers, are bulky and they involve movingparts that can reduce the reliability of the overall system. If theoperating temperature of the detector array can be increased to 200° Kand higher, it approaches the range of temperatures that can be attainedby thermoelectric (TE) coolers that do not involve moving parts. If theoperating temperature can be increased even to 150-200° K, it can becooled by radiative means for imagers used in space. Thus, there is aneed for infrared detector arrays that can operate with low noisecurrent at temperatures of 150° K and higher.

The noise current of an un-illuminated infrared detector, or its darkcurrent, has several major components. One component is ageneration/recombination current (G/R current) that is limited by G/Rcenters at material interfaces such a homojunctions or heterojunctionsin the detector. Another component is a diffusion current that, for highquality materials, is limited by thermal generation in the bulk of thelight-absorbing material. Yet another component is asurface-recombination current due to interface electronic statesresulting from un-passivated dangling chemical-bonds at the outerboundaries of the detector semiconductor material. For many commoninfrared detector materials, such as HgCdTe and antimony-basedcompounds, the G/R current typically dominates the dark current at lowtemperatures, such as below 120-150K. However, at higher temperatures,the diffusion current and the thermal generation current within the bulkabsorber regions dominate the dark current.

One way to reduce the ratio of diffusion current to G/R current at thehigher operating temperatures is to reduce the volume of the absorbermaterial. However, this reduction of absorber volume typically alsoresults in a reduction of the photon absorption efficiency or quantumefficiency of the infrared detector. The disclosed detector achievesboth reduced diffusion current as well as high quantum efficiency topermit operation at higher temperatures. The reduced diffusion currentis accomplished by reducing the volume of absorber material, for a giveninput cross-sectional area of detector array or, alternatively, a givenpixel area. The high quantum efficiency is achieved by using geometricalfeatures that greatly reduce the net front-side reflection of theincident light and also that trap the incident light such that the lightmakes multiple passes through the absorber regions.

FIG. 6 shows results from calculations of the absorbance (normalizedabsorption) of a detector comprising a uniform thickness of MWIR(MidWave Infra Red) absorbing material for various wavelengths ofincident light. Such a detector structure is representative of the mesastructures of FIGS. 1 and 2 as well as the planar structure for theshorter-wavelength detecting portion of FIG. 1. These calculationsassumed an InSb light-absorbing material but the disclosed detectorarray actually could comprise any light-absorbing material that absorbslight at the desired range of wavelengths for its two detection bands.The results indicate that the thickness of the absorbing layer should beat least twice the wavelength of the incident light in order to achievemaximum broadband absorbance. For thinner absorbing layers, there arestrong oscillations in the dependence of absorbance on layer thicknessthat can be associated with multi-pass optical cavity effects(Fabry-Perot cavity resonances). Since it is desirable to reduce thevolume of absorber material, a typical prior detector could have a metalreflector located at the side of the detector opposite the incidentlight. This metal reflector has greater reflectance that the reflectionassociated with just the high refractive index of the light-absorbingmaterial, thereby improving the detector quantum efficiency, especiallyat the wavelengths of the Fabry Perot resonances.

The results in FIG. 6 also show that the absorbance obtained for verythick layers of absorber approaches a value of approximately 0.6. Thislow absorbance is due to the front-surface reflection of the light,since the refractive index of the absorber material is much largerthan 1. Many prior detectors have anti-reflection (AR) coatings thatcomprise one or more layers of dielectric films that have the desiredcombination of film thickness and refractive index to minimize thereflection at specific wavelengths of the incident light. However, it isdifficult to obtain AR coatings that are suitable for a wide range ofincident wavelengths, such as a range approaching one octave. Also, itis difficult to obtain AR coatings for dual-band detection unless thewavelengths of those two bands are multiples of each other. Thus, itwould be very difficult to obtain an AR coating that provides lowreflection at 0.9-1.6 micron wavelength as well as at 3.0-5.0 micronwavelength, for example. There remains a need for a multi-band infrareddetector that has low front-side reflection as well as high externalquantum efficiency at all detected bands.

BRIEF DESCRIPTION OF THE INVENTION

One aspect regarding the presently disclosed technology is that theabsorber regions comprise discrete islands of light-absorbing materialhaving a geometric shape and preferably a trapezoidal shape and thus thevolume of that light absorber is reduced substantially compared absorbervolume of a detector having a uniform-thickness absorber layer.

In one aspect the present invention relates to an array ofphotodetectors respective to incident light comprising: a substrateformed of a material which is transparent to said incident light which,in use, impinges a first major surface of said substrate; eachphotodetector in said array comprising: a plurality of multiple discreteregions of a first optical absorber material, each discrete regionhaving a geometric shape which it shares in common with the otherdiscrete regions of the first optical absorber material, the commongeometric shape having sloping sidewalls which angle towards a tip ofsaid common geometric shape, said plurality of multiple discrete regionsof the first optical absorber material being disposed on or adjacent asecond major surface of said substrate; and a separate metal reflectorand contact layer which contacts or adjoins the tips of the multiplediscrete regions of the first optical absorber material.

In another aspect the present invention relates to a method of making anoptical detector, the method including: forming a first optical absorbermaterial on or adjacent a substrate; etching recesses into the firstoptical absorber material to define a set of multiple discrete absorberregions formed of said first optical absorber material, each discreteabsorber region having a geometric shape which it shares in common withthe other discrete absorber regions, the common geometric shape havingsloping sidewalls which angle towards a tip of said common geometricshape, a combined volume of the discrete absorber regions being smallerthan a volume of etched recesses; and forming a metal reflector incontact with the tips of the discrete absorber regions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b depict a prior art dual band detector with twoindependent electrical contacts for extracting the photo-currents fromthe two bands.

FIGS. 2 a and 2 b depict a prior art dual band detector with oneindependent contact shared by both P/N diodes of a given pixel.

FIG. 3 depicts a reduced volume single-band detector with pyramid shapesetched into the light-absorbing layer.

FIG. 4 shows a prior art solar cell with pyramid-shaped features etchedinto its substrate.

FIG. 5 shows a prior art solar cell with shaped absorber formed above ashaped substrate.

FIG. 6 is a graph showing the dependence of the absorbance of auniform-thickness absorbing layer on the layer thickness and thewavelength of the incident light FIG. 7 is a side elevation schematicview of a portion of an infrared focal-plane array imager with two (ofmany) detector pixels shown.

FIG. 8 shows the detector array of FIG. 7 coupled to a read out circuit.

FIG. 9 a is the top view and FIG. 9 b is the bottom view of detectorstructure with pyramidal anti-reflection structures and trapezoidalabsorber regions, but with differing numbers of pyramidalanti-reflection structures and trapezoidal absorber regions per pixel.

FIG. 10 illustrates the volume fill ratio (VFR) of MWIR absorbermaterial for several detector structures.

FIGS. 11 a-11 h depict preferred steps in the fabrication of thedetector array of FIG. 6.

FIG. 12 provides examples of models for EM simulations of the detectorpixel.

FIG. 13 provides examples of models for EM simulations of structureswith curved sidewalls.

FIGS. 14 a-14 c show the calculated absorbance of exemplary MWIRdetector structures with (FIG. 14 a) zero gap between absorber regionson a square grid, (FIG. 14 b) 1.5 micron gap between staggered absorberregions and (FIG. 14 c) 3.0 micron gap between staggered absorberregions.

FIGS. 15 a-15 c show additional steps which may be added to thedisclosed fabrication steps to provide a detector array having responseat Visible/NearinfraRed wavelengths.

FIG. 16 shows an embodiment where the detector array structure can beused to realize detectors with reduced absorber volume that use acontinuous layer to absorb the incident light to be detected

DETAILED DESCRIPTION

In one embodiment, the present invention is a photo-detector array foran infrared focal-plane array imager. The main features of the discloseddetector array are illustrated in FIG. 7 which is a side sectional viewof two detector pixels 100 of many such detector pixels 100 in an arrayof such detector pixels of an infrared focal-plane array imager. A sidesectional view of the array taken at a ninety degree angle to thesection depicted by FIG. 7 is basically identical to the view of FIG. 7,except that the common contact 170 for the array may be omitted sinceonly one such contact is required (although multiple such contacts maybe utilized if desired, and thus it may appear in a side sectional viewof the array taken at a ninety degree angle to the section depicted byFIG. 7.

The detectors (or detector pixels or photodetectors) 100 of this arraycomprises multiple discrete or separated regions 120 of a first layer oflight-absorbing material 120′ (see FIG. 11 a) disposed either on asubstrate 110 that is transparent to the incident light 200 to beabsorbed or on a layer 150 of layer of light-absorbing material disposedon the substrate 110. These first light-absorbing material regions 120formed in layer 120′ preferably have a trapezoidal shapes (when viewedin a side view such as FIG. 7) but could alternatively have pyramidal orconical shapes. A trapezoidal shape, with a flattened pointed end, ispreferred as the flattened pointed end should make a better electricalcontact with contact 130 and, moreover, a trapezoidal shape tends toresult when trying to form a pyramidal shape at the feature sizesinvolved in these detectors. The substrate 110 comprises material thatpreferably is transparent to all wavelengths of the incident light 200to be sensed by each detector 100. The first light-absorbing materialregions 120 are located on or near the side 110 b of the substrateopposite the side 110 a facing the incident light. Each detector 100also comprises a metal reflector and electrical contact 130 that isdisposed immediately adjacent the tips 120 a of the multiple trapezoidallight-absorbing regions 120. The side 110 a of the substrate 110 facingthe incident light 200 has other pyramid shaped features 140 formed init (these features 140 appear as triangles in the side elevational viewof FIG. 7). Thus, the photodetector 100 has geometric shapes formed onboth of its opposing sides. As discussed below, there also is a portionof the substrate into which the pyramid shapes do not extend.

Each photodetector 100 preferably has a second layer of light-absorbingmaterial 150 whose absorption cut-off wavelength is shorter than theabsorption cut-off wavelength of the material comprising the multiplediscrete regions 120. The light absorbing material 150 may be disposedbetween the substrate 110 and the light absorbing material 120. Theseportions of differing light-absorbing materials may be separated by abarrier layer 160 that passes minority carriers but blocks majoritycarriers generated in the light-absorbing layers 120, 150. The firstlight-absorbing regions 120 are formed from a layer of the firstlight-absorbing material that has deep trapezoid-shaped recesses 124formed in it. These trapezoid-shaped recesses 124 preferably extendcompletely through that layer of first light-absorbing material 120 (andpreferably into layer 150 thereby forming recesses 122 therein), so thatfirst light-absorbing material layer forming regions 120 isdiscontinuous, thereby forming multiple, discrete absorbing regions 120.In FIG. 7, the trapezoid shaped recesses 124 extend completely throughthe barrier layer 160 and into the second layer of light-absorbingmaterial 150. Alternatively, the trapezoidal shaped recesses 124 couldextend only through the first light-absorbing layer and then stop at thebarrier layer 160 such that the barrier layer remains continuous.Another alternative has the trapezoid-shaped recesses 124 stop atsubstrate 110, especially if layer 150 is omitted.

The first layer 120 may have a material that absorbs light of a firstwavelength band, such as the mid-wave infrared (MWIR) wavelengths. Thesecond layer 150 may have material that absorbs light of a secondwavelength band, such as the near infrared (NIR) and/or short-waveinfrared (SWIR) wavelengths, but that is nominally transparent to thelight of the first wavelength band. Without implying a limitation, NIRis approximately 0.7 to 1.4 microns, SWIR is approximately 1.4 to 3.0microns and MWIR is approximately 3.0 to 8.0 microns.

Continuing with FIG. 7, each detector pixel 100 has an individualelectrical contact 130, with the lateral extent of that electricalcontact defining the spatial extent of the pixel detector 100. FIG. 7also shows a portion of the array containing the common electricalcontact 170. The embodiment of FIG. 7 illustrates two-band detectors 100that absorbs light of MWIR and SWIR wavelengths. Each detector 100 hasmultiple regions of MWIR absorber material 120 and a region of SWIRabsorber material 150. The MWIR absorber in a given pixel preferablyconsists of multiple discrete regions that preferably have a trapezoidalshape, although other shapes such as pyramids and cones also aresuitable. The SWIR absorber region of a pixel is continuous but may haverecesses 122 formed in it, as illustrated by FIG. 7, due to theformation of the aforementioned trapezoid-shaped recesses 124. The SWIRabsorber 150 and MWIR absorber 120 are preferably separated by barrier160. Barrier 160 preferably has an electronic band structure that passesminority carriers of at least the MWIR absorber and possibly also of theSWIR absorber but that blocks the majority carriers of both absorbers,and forms heterojunctions with the absorber materials. Alternatively,barrier 160 could be obtained by doping such that it forms a NPN or PNPback-to-back diode structure with the SWIR 150 and MWIR 120 absorbermaterials. The barrier 160 of a pixel could comprise multiple discreteregions, as illustrated in FIG. 7, or it could comprise a continuousregion, as mentioned above. The substrate 110 consists of a materialthat is preferably transparent to the wavelengths of light to bedetected by the SWIR and MWIR absorber regions 150, 120. The material ofthe SWIR absorber 150 is transparent for the wavelengths of light to bedetected by the MWIR absorber regions 120. The SWIR absorber 150, incombination with barrier 160, serves as a transparent collector for theMWIR detector to extract the carriers photo-generated in the MWIRabsorber regions 120. Also, the MWIR absorber regions 120, incombination with barrier 160 serve as collector regions for the SWIRdetector to extract the carriers photo-generated in the SWIR absorber150. Each detector pixel 100 also preferably contains regions ofmaterial 180 that provides electronic surface passivation for theexposed surfaces of the absorber and barrier regions. This surfacepassivation material 180 also could fill in the voids 124 between theMWIR absorber trapezoids 120, including the optional recesses 122 formedin the SWIR absorber 150, as illustrated in FIG. 7. Alternatively, thesurface passivation 180 material could comprise a thin film thatconformally covers the exposed surfaces of the absorber and barrierregions.

The substrate 110 is located closest to the incident light 200 and isfollowed, in succession, by the SWIR absorber 150, the barrier 160 andthen the MWIR absorber regions 120. The side of the substrate facing theincident light preferably has a texture 140 etched into its surface 110a. The surface texture can be a set of pyramids, as illustrated by thetriangular shapes in the side elevation view of FIG. 7. Other suitablesurface textures include pillars, cones, ridges, holes and invertedpyramids and cones. The surface texture 140 acts to provideanti-reflection of the incident light 200 for a wide range ofwavelengths of light that includes the wavelengths absorbed by the SWIR150 and MWIR 120 absorbers. The surface texture 140 also serves toslightly deflect the angle of the light coupled into the substrate andtoward the absorbers. Each detector pixel 100 also includes a broadbandreflector such as a metal reflector 130 that is located at the tips 120a of the trapezoidal absorber regions 120. The reflector 130 preferablyhas a flat surface rather than a surface that conforms to the sides ofthe trapezoids (that is, the tips 120 a of the trapezoidal absorberregions 120 may penetrate into reflector 130 slightly but preferablywithout having reflector 130 conform to the sides of the trapezoidalabsorber regions 120). Also, a portion of the reflector 130 ispreferably spaced away from the trapezoidal absorber regions 120 andthereby defines one or more voids 124 between adjacent trapezoidalabsorber regions 120. The refractive index of the absorber materials120, 150 is much higher than the refractive index of the surfacepassivation 180 or of any voids 122, 124 between the trapezoidal regions(if either no surface passivation 180 is applied or if the surfacepassivation 180 is a thin film as previously mentioned). The geometry ofthe detector 100 structure enhances the trapping of incident light suchthat the light makes multiple passes through the regions 120 of the MWIRabsorber and even through the SWIR absorber 150, preferably many morethan two passes. This increased number of passes of the light to beabsorbed can cause the disclosed detector to have better quantumefficiency per unit volume of absorber material than the prior detectorsdiscussed above. Light that is not absorbed on a first pass through theabsorber regions can be reflected by the metal reflector 130 as well asdeflected and/or reflected by the sloped walls of the trapezoidalabsorber regions 120 or by the sloped walls of the recesses 122 suchthat the light makes additional passes through the absorber regions 120,150.

Additional details regarding the disclosed detector array are shown anddiscussed with reference to FIG. 8. As previously mentioned, eachdetector pixel 100 has an electrical contact 130, which preferably is anohmic contact, that is formed at (immediately adjacent) the tips 120 aof the trapezoidal absorber regions 120. This electrical contact 130provides extraction of the photo-generated carriers resulting from thelight detected by the associated detector pixel 100. A cap layer 125 ofmore heavily doped material of relatively small bandgap may be grown onthe MWIR absorber layer forming regions 120 so that the tips 120 a ofthe trapezoidal absorber regions 120 may be formed by this cap layer125. The detector array also can include a second set of electricalcontacts 170 that are made to the SWIR absorber 150. This second set ofcontacts 170 serve as the array-common contacts and generally are commonto multiple pixels 100 of the array. Regions of heavily doped material152 may be formed in the SWIR absorber 150 at these contacts tofacilitate the achievement of ohmic contacts. The detector array alsomay include metal plugs 250 and bond pads 240 as well as solder bumps230 that are used to provide electrical connections between the array ofdetectors 100 and a read out integrated circuit 220. A focal plane arrayimager could thus include the disclosed detector array coupled to aread-out integrated circuit 220.

In order to obtain good anti-reflection for all wavelengths of light tobe detected, the width of the pyramids of the texture 140 etched intosurface 110 a of substrate 110 preferably are as large as or smallerthan 2-times the shortest wavelength of the light to be absorbed by thedetector pixels 100. Thus, if the shortest wavelength of absorbed lightis 1.5 micrometers, the width of the pyramids at their base should be atmost 3 micrometers. The height of the pyramids of the texture 140 etchedinto surface 110 a of substrate 110 is preferably on the order of thelongest wavelength of the light to be absorbed. Thus, if the MWIR lightto be absorbed has a 3-5 micrometer wavelength, the height of thepyramids should preferably be around 3 to 5 micrometers. Thesedimensions for the height and width are by way of example and notlimitation. The pyramids of the texture 140 etched into surface 110 a ofsubstrate 110 form a continuous pattern on the side of the substrate ofthe detector facing the incident light, as illustrated in FIG. 9 a. Agiven detector pixel 100 will include many pyramidal structures in layer140. Those pyramids can be arranged in a staggered manner, asillustrated in FIG. 9 a. Alternatively, those pyramids may be arrangedin any other pattern or formed in any other shape that provides adesired anti-reflection and photon retention property.

The size of the trapezoidal absorber regions 120 is generally largerthan the size of the pyramids of the AR surface texture 140. The widthof the tip 120 a of each absorber region 120 should be smaller than theshortest wavelength of the MWIR band to be absorbed. The width of thebase 120 b of each trapezoidal absorber region 120 is on the order ofthe longest wavelength of the MWIR band to be absorbed. The height 120 cof each trapezoidal region 120 preferably is equal to or greater thanthe longest wavelength of the MWIR band to be absorbed. Thus, forexample and not to imply a limitation, if the MWIR band is 3-5 microns,the trapezoid may have a 5 micron base width, a 2 micron tip width and a5 micron height 120 c.

The bases 120 b of adjacent trapezoidal absorbers 120 may touch eachother such that both the barrier 160 and the SWIR absorber layer 150 arecontiguous (as illustrated in portion (b) of FIG. 10) if no recesses 122are formed in the SWIR absorber 150. For the example mentioned above inthe preceding paragraph, this means the center-to-center spacing betweenof each trapezoidal absorber region 120 would be about 5 microns.Alternatively, the bases of adjacent trapezoidal absorber regions 120will have gaps or spaces between them such that their bases do not toucheach other when recesses 122 are formed in the SWIR absorber layer 150.This configuration is illustrated in FIGS. 7, 8, 9 b and portion (c) ofFIG. 10. In this case, the trapezoidal MWIR absorber regions 120preferably are the upper portions of larger trapezoids that also includethe barrier 160 and part of the adjacent SWIR absorber 150. As thetrapezoidal MWIR absorber regions are located farther and farther apart,the volume fill ratio (VFR) of the MWIR absorber 120 is reduced. Auniform thickness absorber layer (illustrated in portion (a) of FIG. 10)has a volume fill ratio of 1.0. A detector structure with trapezoidalMWIR absorber regions 120 whose bases touch has a volume fill ratio of0.35 to 0.45 (depending on the specific dimensions of the trapezoids). Adetector structure with trapezoidal MWIR absorber regions 120 whosebases do not touch can have a volume fill ratio of 0.35 to 0.1, orsmaller.

The array of detector pixels 100 has electrical contacts 130, definingthe individual pixels, that are made to the MWIR absorber regions 120.The contact 130 for a given pixel 100 can be connected to multipletrapezoidal region tips 120 a, and there can be many trapezoidal regionsof MWIR absorber 120 associated with one pixel 100 as illustrated inFIG. 9 b.

One possible fabrication process for the disclosed array of detectors100 begins with the epitaxial growth of the SWIR absorber layer, thebarrier and the MWIR absorber layer on an appropriate substrate wafer110. See FIG. 11 a. One example of a suitable substrate material 110 isGaAs onto which a SWIR absorber material 150, such as InAlAsSb orInGaAsSb, is grown. The barrier layer 160 may comprise combinations ofmaterials such as AlAsSb, GaAlSb, or InGaAlSb. The MWIR absorber regions120 may comprise materials such as InAsSb, InSb, or InGaAsSb. Theabsorber layers 120, 150 are preferably un-intentionally doped(preferably undoped) or lightly doped. The barrier 160 could beun-intentionally doped (preferably undoped) or intentionally doped. Thecap layer 125 of a more heavily doped material of relatively smallbandgap may be grown above the MWIR absorber layer 120′ (see FIG. 11 a)used to form MWIR absorber regions 120. This cap layer 125 facilitatesthe formation of ohmic contact between the MWIR absorber regions 120 andthe metal contacts 130.

Alternatively, the HgCdTe family of alloys can be utilized to fabricatethe detectors formed by the absorber materials in layers 120′, 160. Byadjusting the mole fraction (X) of Hg_(1-X)Cd_(X)Te, the bandgap can beadjusted to cover wavelengths from the SWIR to the VLWIR (Very Long WaveInfraRed) bands. HgCdZnTe alloys can be utilized as the barrier 160layer.

The detector array fabrication steps, which are illustrated by FIGS. 11a-11 h, include the patterned etching of the trapezoidal shapes todefine the regions 120 in the MWIR absorber layer 120′. First, recesses124 are etched into a MWIR absorber layer 120′ which recesses may extendbeyond the MWIR layer 120′, penetrating through the barrier 160 and intothe underlying SWIR absorber layer 150 to form recesses 122 therein asshown, for example, in FIGS. 7, 8, and 11 b. The sloping sidewalls ofregions 120 in the MWIR absorber layer 120′ preferably continue assloping sidewalls which penetrate into the underlying SWIR absorberlayer 150, each sloping sidewall which penetrates into the underlyingSWIR absorber layer 150 preferably sharing a common plane with theadjacent sloping sidewall of the adjacent region 120. Etched recesses ofvarious sidewall angles as well as sidewall curvatures can be obtainedby combinations of dry etching (such as reactive ion etching or ionmilling) and wet-chemical etching processes. The exposed surfaces ofMWIR and SWIR absorber material and barrier material 160 can be coatedwith a surface passivation or dielectric layer 180. The passivationlayer 180 can be a thin film or it can be thicker and completely orpartially fill the voids 122, 124 between adjacent trapezoidal shapedregions 120. See FIG. 11 c. In one embodiment, the etched recesses 122,124 can be filled with a low-index dielectric material 180 such assilicon dioxide, polyimide or benzo-cyclobutene. Electrical contacts canbe made to the tips 120 a of the MWIR absorber regions 120 (and to thecaps 125 if formed at the tips 120 a) by making sure no dielectric oflayer 180 covers the tips 120 a of the trapezoidal regions and then bypatterned deposition of appropriate contact metals 130 onto the detectorwafer. See FIG. 11 d. Materials such as TiAu and TiPtAu could be usedfor these metal contacts. The contact metal 130 also preferably servesas an optical reflector as previously disclosed.

The detector array also has a common electrical contact 170 that isshared by multiple detector pixels 100. This contact 170 makeselectrical connection to the SWIR absorber layer 150. If layer 150 isnot used, then contact 170 is made to the substrate 110 instead andrecesses 122 may then penetrate into substrate 110. The steps related tofabrication of this common contact 170 are now discussed with referenceto FIGS. 11 e-11 h. These steps include removal of the MWIR 120′ andbarrier material 160 from the vicinity of that common contact 170 asneeded to expose the underlying SWIR absorber layer 150. See FIG. 11 e.Also, heavily doped regions 152 (see also FIG. 8) can be formed in theexposed SWIR absorber by means such as ion implantation and annealing tofacilitate the fabrication of ohmic contact to the wider bandgap SWIRabsorber material 150. Then, contact metals 154 such as TiAu, TiPtAu orAuGeNiAu can be deposited onto the exposed (and preferably doped) regionof SWIR absorber. See FIG. 11 f. A metal plug 250 also can be formed bymeans such as electroplating to make the top of the plug coplanar withthe metal pads for the individual-pixel contacts. See FIG. 11 g. Thisfacilitates connection of the detector array to a readout circuit 220using solder bumps 230. See FIG. 11 h.

Before mounting to a readout circuit 220, the detector wafer can bemounted temporarily onto a carrier wafer (not shown) such that the backside 110 a of the detector substrate (the transparent substrate 110) isexposed. The substrate 110 can then be thinned, as needed, to a desiredthickness and can be patterned for etching of the AR texture 140(preferably formed by the previously mentioned pyramid shapes 140) intothat back side, as also illustrated in FIG. 11 h. After the AR texture140 is etched, an optional layer (not shown) of material, such as a hightemperature wax, resin or removable polymer, can be temporarilydeposited onto that back side 110 a to protect the texture 140 fromphysical damage during subsequent manufacturing operations. Thecompleted detector wafer can then be demounted from the carrier waferand mounted onto an appropriate readout circuit 220, by known means suchas the aforementioned solder bumps 230. The temporary protective coatingdeposited on the back side 110 a of the substrate 110 can be removedafter the detector array has been mounted onto the readout circuit 220.

FIGS. 9 a and 9 b show, respectively, staggered arrangements of theanti-reflection pyramid shapes of layer 140 and the trapezoidal absorberregions 120. However other arrangements also are possible, such asgrid-like (non-staggered) arrangements or even somewhat randomarrangements. Examples of a grid-like arrangement and a staggeredarrangement are shown in FIG. 12. Note that for these regulararrangements, there can be a base repetition period of the geometricstructures. FIG. 12 shows examples of a basic structure or unit cellthat can be repeated or arrayed to produce the overall structure of thedetector. This basic structure can be used as a model forelectro-magnetic simulations that analyze the optical characteristics ofthe detector. For example, it is possible to calculate the netfront-surface reflectance of the incident light and the net absorbanceof the light, for various wavelengths of that light. These calculationscan be made for various dimensions of the pyramid 140 and trapezoid 120and recess 190 as well as various values of the substrate 110 thickness.

The sidewalls of the pyramids and the trapezoids could be curved ratherthan flat, as illustrated in FIG. 13. Also, those sidewalls could becurved inward or curved outward. The effect of sidewall curvature on thefront-side reflection and on the photon capture by the detector is morepronounced for the relatively shorter wavelengths. The pyramids and thetrapezoids also could be asymmetric. For example different sidewallscould be tilted at different angles. Or, some sidewalls could be flatand other sidewalls could be curved.

Embodiments like those shown in FIGS. 12 and 13 can be used to calculatethe effects of the arrangement of the trapezoidal absorbers and of thespacing between adjacent absorber regions (which affects the volume fillratio) on the net absorbance of a detector. In this way, one candetermine the effect of absorber geometry and arrangement on the valueof a figure of merit such as the net absorbance per unit volume ofabsorber material. For diffusion-current limited noise, that absorbanceper unit volume gives an indication of the anticipated signal-to-noiseperformance of the detector. For a prior-art detector that has anabsorber layer of uniform thickness, the thickness of that absorberlayer often is chosen to maximize its absorbance per unit volume figureof merit. For the disclosed detector, additional parameters such as thespacing, bandwidth and sidewall angles of the shorter-wavelength (e.g.,MWIR) trapezoid-shaped absorber regions, the angle of the pyramids inthe substrate and the shape of the recesses in the longer-wavelength(e.g., SWIR) absorber layer also could be selected to maximize theabsorbance per unit volume figure of merit, for some given wavelengthbands and incidence angles of the light to be detected.

FIGS. 14 a-14 c show examples of the calculated absorbance obtained withseveral exemplary arrangements of the multiple discrete absorbers andthe pyramid-shaped AR texture 140 with the calculated absorbance ofexemplary MWIR detector structures with (a) zero gap between absorberregions on a square grid (see FIG. 14 a); (b) 1.5 micron gap betweenstaggered absorber regions (see FIG. 14 b); and (c) 3.0 micron gapbetween staggered absorber regions (see FIG. 14 c). The calculationswere done using HFSS, a 3D electromagnetic field simulation and analysistool from Ansoft Corporation. The results, obtained for an exemplaryInSb MWIR absorber illustrates the effect of the spacing between thediscrete absorber regions 120, which have a pyramid shape in this case.It also illustrates the effect of the geometric arrangement of theabsorber regions. The absorbance of the detector is quite high, above90%, even though the volume of absorber material is reduced to 0.33 andeven to 0.08 the volume of a uniform-thickness absorber layer. So thevolume of voids 124 is then preferably between 0.67 and 0.92 of thetotal volume of both the voids 124 and the MWIR absorber regions 120.For these cases, the smaller volume fill ratio was obtained by keepingthe dimensions of the MWIR absorber regions 120 the same but increasingthe spacing between them. For the structures in which the bases of theMWIR absorber regions do not touch each other, those absorber regionsactually comprise only the upper portions of larger pyramids that extendinto material that do not absorb the MWIR light. When compared to theresults shown in FIG. 6, the results of FIGS. 14 a-14 c indicate thatthe disclosed detector is effective in reducing the absorber volume, andthus the dark current noise of the detector, while also achieving highabsorption efficiency for incident light over a broad range ofwavelengths.

The photo-detecting element or pixel 100 of the disclosed photo-detectorarray has reduced volume of the photon-absorber material for achievingreduced dark current (and improved noise performance) at temperatureshigher than cryogenic temperatures (e.g., at temperatures of 150-200° Kand higher, depending on the cut-off wavelength of its light-absorbingmaterial). The photo-detecting element has geometric features thatincrease the capture and retention of incident photons (especially thelonger-wavelength light) so that the quantum efficiency of thephoto-detector is high even though its absorber volume is reduced. Thephoto-detecting element also contains geometric features that reduce thenet front-side reflection of the incident light, over a largemulti-octave range of photon energies or optical wavelengths so that lowreflection is obtained even for detectors that operate in multipleoptical-wavelength bands (such as at both the entire SWIR and the entireMWIR bands). One aspect of the presently disclosed detector is that itspreferably trapezoidal-shaped absorber regions 120 comprise discreteislands of a light-absorbing material and the volume of those islands oflight-absorbing material is reduced substantially compared to theabsorber volume of a detector with a uniform-thickness absorber layer.

ALTERNATIVE EMBODIMENTS

The embodiments described above are only exemplary. Many othervariations beyond those already mentioned are possible. For example,although those embodiments contain multiple absorber regions for thelonger-wavelength detection band that are separated, it also is possiblefor the disclosed detector to have an absorber that is not discrete butrather that has the trapezoidal regions 120 (or regions of other shapes)connected together. The VFR would be higher in that case but a higherVFR could still be acceptable for some applications (such as thoseinvolving lower operating temperature and/or absorber material withshorter cutoff wavelength). Also, the SWIR absorber region 150 depictedin the figures could comprise the same material as the substrate 110. Inthat case, the detector could be used to detect light of a singleinfrared band rather than two bands. Additionally, the metal reflector130 need not be a flat sheet but rather could have curved or slopingregions. The main requirement is that the metal reflector 130 layertouches the electrical-contact regions (tips 120 a) of the multipleabsorber regions 120, since that metal layer also provides theelectrical connection between those absorber regions 120 for themajority carriers in those regions.

The embodiments described above have a substrate whose material is GaAs.GaAs is transparent for optical wavelengths of 0.9 microns and longer.Thus, the embodiments set forth above as given are for a firstlight-absorbing material 120 that absorbs MWIR light and a secondlight-absorbing material 150 that absorbs SWIR light. It is possible toobtain a detector that also detects near-infrared (NIR) wavelength lightand/or visible wavelength light by using a substrate material that istransparent to those wavelengths of light. Examples of such substratesare GaN, SiC and sapphire. High quality substrate wafers of those wideband-gap materials are available from commercial suppliers.

First and second absorber materials made from antimony-based compoundscan be epitaxially grown on GaAs substrates. However, growth of theantimony-based compounds on GaN, SiC and sapphire is not a welldemonstrated process. One way to form a detector that has layers of theantimony-based materials formed on a wide band-gap substrate is nowdescribed with reference to FIGS. 15 a-15 c.

FIGS. 15 a-15 c illustrates a process in which those absorber andbarrier layers are bonded onto a wide band-gap substrate of GaN, SiC orsapphire. First, a stop-etch layer 310, a layer of the firstlight-absorbing material 120, the barrier layer 160 and then a layer ofthe second light-absorbing material 150 are grown on a sacrificialsubstrate 300, such as GaAs. In addition, an optional cap layer ofheavily doped first light-absorbing material could be between thestop-etch layer and the layer of first light-absorbing material. Thesubstrate with the grown epitaxial layers can then be wafer bonded ontothe wide band-gap substrate 110 _(wbg) using a known wafer bondingprocess, such as direct contact between surfaces that are very flat andclean. Those two surfaces are held together by Van der Waals forces.After the bonding, the sacrificial substrate 300 can be removed bythinning that substrate and by etching it away with a selective etchantthat attacks only the substrate material 300 but not the material of thestop-etch layer 310. For example, the stop-etch layer 310 could consistof a GaAlSb-based material. There are known wet-chemical etchants anddry etching processes that will etch GaAs but that will not etch thealuminum-containing compound. Finally, another etchant can be used toremove the stop-etch layer 310 and not etch the first light-absorbingmaterial 120.

The remaining fabrication steps for this detector, which also hasresponse at Visible and NIR wavelengths, are preferably similar to thesteps shown in FIGS. 11 b-11 h. The short-wavelength cut-off of thedisclosed detector is thus determined by the transparency cut-off of thesubstrate material 110 _(wbg). The long-wavelength cut-off of thedisclosed detector is determined by the absorption cut-off of the firstlight-absorbing material. This absorption cut-off wavelength isdependent on the band gap of that first light-absorbing material.

A dual-band detector with a wide band-gap substrate actually could haveits second band respond to Visible-SWIR, Visible-NIR or Visible onlywavelengths. The absorption cut-off wavelength of that secondlight-absorbing material determines the long-wavelength edge of thatsecond band. The second light-absorbing layer can be made sufficientlythick that all of the light of that second detection band is absorbed ona single pass through that layer, and thus it does not penetrate intothe first light absorbing layer. In this case, that absorption cut-offwavelength also determines the short-wavelength edge of the firstdetection band.

Besides having the wafer bonding occur directly between the finalepitaxially grown layer of the detector and the wide band-gap substrate,such wafer bonding could be facilitated by an intermediate layer orlayers of other materials such as silicon dioxide or benzo-cyclo-butene(BCB). When silicon dioxide is used as the intermediate layer, apreferred fabrication approach is to deposit a thin film of silicondioxide on the final epitaxially grown layer (such as on a layer ofsecond absorber material 150) and another thin film of silicon dioxideon a smooth surface of the wide band-gap substrate. The thickness ofthese silicon dioxide films is preferably less than 100 nm. When BCB isused as the intermediate layer, thin films of BCB are deposited on boththe final epitaxially grown layer and the wide band-gap substrate.Alternatively a thin film of BCB can be deposited on either the finalepitaxially grown layer or the wide band-gap substrate. The totalthickness of the BCB layers is preferably less than 200 nm.

The wafer bonding is done by pressing the clean silicon dioxide films(or the BCB films) together, preferably in an evacuated chamber. Also,the wafer bonding could be assisted by heating the two bonded pieces toa temperature of several hundred degrees-C. or higher. The resultingsilicon dioxide layer or BCB layer forms a thin wafer-bond interfacelayer.

The combination of the wafer-bond interface layer surrounded by thefinal epitaxially grown layer and by the wide band-gap substrate can actlike an optical Fabry Perot cavity. The refractive index of thewafer-bond interface layer is fairly low, approximately 1.5. Incomparison, the refractive index of the surrounding materials is muchhigher, being approximately 3.0-3.5 for the second absorber material andapproximately 1.7-1.8, 2.55-2.7, and 2.25-2.35 for sapphire, siliconcarbide and gallium nitride, respectively. It is important to keep thethickness of the wafer-bond interface layer sufficiently small that thisoptical cavity minimally reflects the incident light to be detected. Forexample, if the shortest wavelength of the incident light to be detectedis 450 nm, the thickness of the wafer-bond interface layer can be450/2/1.5=150 nm. The resulting optical cavity acts as ananti-reflection structure for the 450 nm light and has moderate to lowreflection for light of longer wavelengths. However, in terms ofminimizing the wideband optical reflection resulting from the wafer-bondinterface, it is preferable to have a direct bond between the finalepitaxially grown layer and the wide band-gap substrate, rather than usean intermediate interface layer. However, such interface layers canresult in a bond that is easier to fabricate or that is more robust.

The disclosed detector array structure also can be used to realizedetectors with reduced absorber volume that use the continuous layer 150to absorb the incident light to be detected. In this embodiment, whichis depicted by FIG. 16, the thickness of layer 150 is generally notsufficient to completely absorb the incident light in a single pass ofthat light through layer 150. For example, a detector of MWIR wavelengthlight could be obtained by making layer 150 from a MWIR absorbingmaterial, examples of which are given above. The trapezoidal regions 120are then preferably made from a material that is transparent to the MWIRlight. For example, the trapezoidal regions could be made from the SWIRabsorbing material, examples of which are given above. Although thosetrapezoidal regions 120 do not absorb the incident light to be detected,they have a high optical refractive index and thus are effective fortrapping the light and increasing the number of passes of that lightthrough the absorber layer 150. The regions 120 then serve asMWIR-transparent collector regions 120 to extract the minority carriersthat are generated in the MWIR absorber layer 150.

What is claimed is:
 1. An array of photodetectors responsive to incidentlight comprising: a. a substrate formed of a material which istransparent to said incident light which, in use, impinges a first majorsurface of said substrate; b. each photodetector in said arraycomprising: (1) a plurality of multiple discrete regions of a firstoptical absorber material, each discrete region having a geometric shapewhich it shares in common with the other discrete regions of the firstoptical absorber material, the common geometric shape having slopingsidewalls which angle towards a tip of said common geometric shape, saidplurality of multiple discrete regions of the first optical absorbermaterial being disposed on or adjacent a second major surface of saidsubstrate; and (2) a separate metal reflector and contact layer whichcontacts or adjoins the tips of the multiple discrete regions of thefirst optical absorber material, said separate metal reflector andcontact layer contacting or adjoining the tips of the multiple discreteregions of the first optical absorber material with a portion of theseparate metal reflector and contact layer being spaced away from themultiple discrete regions and thereby defining one or more voids betweenadjacent ones of the multiple discrete regions, the one or more voidshaving an optical refractive index which is lower than an opticalrefractive index of the first optical absorber material.
 2. The array ofclaim 1 wherein the first optical absorber material includes a cap layerof a more heavily doped material than remaining portions of the firstoptical absorber material and wherein the separate metal reflector andcontact layer contacts or adjoins tips of the multiple discrete regionsformed by said cap layer.
 3. The array of claim 1 wherein the geometricshape of each discrete region appears as a trapezoidal shape when viewedin a side elevation view thereof.
 4. The array of claim 1 wherein thefirst major surface of said substrate has a pyramid shaped featuresformed therein on a surface facing, in use, said incident light.
 5. Thearray of claim 4 wherein the pyramid shaped features formed in saidsubstrate have a smaller feature size than a feature size of thegeometric shape of the discrete regions of the first optical absorbermaterial.
 6. The array of claim 1 wherein regions between the multiplediscrete regions of the first optical absorber material are coated orfilled with a passivation material.
 7. The array of claim 1 wherein: avolume defined between the substrate material and the separate metalreflector and contact layer which does not include the multiple discreteregions of the first optical absorber material defines a first volume,the multiple discrete regions of the first optical absorber materialdefine a second volume, and the second volume is less than said firstvolume.
 8. An array of photodetectors responsive to incident lightcomprising: a. a substrate formed of a material which is transparent tosaid incident light which, in use, impinges a first major surface ofsaid substrate; b. each photodetector in said array comprising: (1) aplurality of multiple discrete regions of a first optical absorbermaterial, each discrete region having a geometric shape which it sharesin common with the other discrete regions of the first optical absorbermaterial, the common geometric shape having sloping sidewalls whichangle towards a tip of said common geometric shape, said plurality ofmultiple discrete regions of the first optical absorber material beingdisposed on or adjacent a second major surface of said substrate; and(2) a separate metal reflector and contact layer which contacts oradjoins the tips of the multiple discrete regions of the first opticalabsorber material, said array further including a second opticalabsorber material disposed between the plurality of multiple discreteregions of the first optical absorber material and the substrate, thesecond optical absorber material absorbing light of a shorter wavelengththan the light absorbed by the multiple discrete absorber regions of thefirst absorber material.
 9. The array of claim 8 wherein the slopingsidewalls of the geometric shape of multiple discrete regions of thefirst optical absorber material continue as sloping sidewalls whichpenetrate into the second optical absorber material, each slopingsidewall which penetrates into the second optical absorber materialsharing a common plane or a sidewall curvature with an adjacent slopingsidewall of an adjacent discrete region of the first optical absorbermaterial to thereby define non-penetrated portions of the second opticalabsorber material which adjoin an associated discrete region of thefirst optical absorber material.
 10. The array of claim 9 wherein: afirst volume is defined between the substrate material and the separatemetal reflector and contact layer which does not include (1) themultiple discrete regions of the first optical absorber material or (2)non-penetrated portions of second optical absorber material a secondvolume is defined by the multiple discrete regions of the first opticalabsorber material, and the second volume is less than the first volume.11. The array of claim 9 wherein a barrier is formed where thenon-penetrated portions of second optical absorber material adjoin anassociated discrete region of the first optical absorber material, thebarrier passes minority carriers but blocks majority carriers generatedin the first and second optical absorber materials.
 12. The array ofclaim 8 wherein the first optical absorber material includes a cap layerof a more heavily doped material than remaining portions of the firstoptical absorber material and wherein the separate metal reflector andcontact layer contacts or adjoins tips of the multiple discrete regionsformed by said cap layer.
 13. The array of claim 8 wherein the geometricshape of each discrete region appears as a trapezoidal shape when viewedin a side elevation view thereof.
 14. The array of claim 8 wherein thefirst major surface of said substrate has a pyramid shaped featuresformed therein on a surface facing, in use, said incident light.
 15. Thearray of claim 8 wherein regions between the multiple discrete regionsof the first optical absorber material are coated or filled with apassivation material.
 16. An array of photodetectors responsive toincident light comprising: a. a substrate formed of a material which istransparent to said incident light which, in use, impinges a first majorsurface of said substrate; b. each photodetector in said arraycomprising: (1) a plurality of multiple discrete first regions of afirst optical absorber material, each discrete first region having ageometric shape which it shares in common with the other discrete firstregions of the first optical absorber material, the common geometricshape having sloping sidewalls which angle towards a tip of said commongeometric shape, said plurality of multiple discrete regions of thefirst optical absorber material being disposed on or adjacent a secondmajor surface of said substrate; (2) a second region of a second opticalabsorber material, the second region being a unitary region disposedbetween each of the plurality of multiple discrete first regions of thefirst optical material and the substrate, the second optical absorbermaterial absorbing light of a shorter wavelength than the light absorbedby the multiple discrete absorber regions of the first absorbermaterial; and (3) a separate metal reflector and contact layer whichcontacts or adjoins the tips of the multiple discrete regions of thefirst optical absorber material.
 17. The array of claim 16 wherein abarrier is disposed between the each of the plurality of multiplediscrete regions of a first optical absorber material and the secondregion of the second optical absorber material, said barrier has anelectronic band structure that passes minority carriers of the at leastthe second optical absorber material but that blocks the majoritycarriers of both the first and second optical absorber materials.
 18. Anarray of photodetectors responsive to incident light comprising: a. asubstrate formed of a material which is transparent to said incidentlight which, in use, impinges a first major surface of said substrate;b. each photodetector in said array comprising: (1) a plurality ofmultiple discrete regions of a first optical material, each discreteregion having a geometric shape which it shares in common with the otherdiscrete regions of the first optical material, the common geometricshape having sloping sidewalls which angle towards a tip of said commongeometric shape, said plurality of multiple discrete regions of thefirst optical material being disposed on or adjacent a second majorsurface of said substrate; (2) a common integral body of a secondoptical material disposed between each of the plurality of multiplediscrete regions of the first optical material and the substrate, thebody of the second optical material absorbing light of givenwavelengths, the first optical material being essentially transparent tosaid given wavelengths; and (3) a separate metal reflector and contactlayer which contacts or adjoins the tips of the multiple discreteregions of the first optical material.
 19. The array of claim 18 whereina barrier is disposed between the each of the plurality of multiplediscrete regions of a first optical material and the second region ofthe second optical material, said barrier has an electronic bandstructure that passes minority carriers of the second optical materialbut that blocks the majority carriers of the second optical material.